Polyesters containing 1,4-cyclohexanedimethanol (abbreviated hereinafter as “CHDM”) as a diol component are often produced using a dialkyl esters such as, for example, dimethylterephthalate (DMT), dimethylisophthalate, and 1,4-dimethylcyclohexane dicarboxylate, as the source of the diacid component. In a typical process, for example, a dialkyl ester is reacted with one or more diols in a transesterification reaction to produce an oligomer. The alkyl alcohol by-product is removed from the reaction medium, usually by distillation, to help to push the reaction equilibrium toward oligomer formation. The transesterification step is conducted at high temperatures, typically from about 200° C. to about 270° C., and at pressures between about 101 to 515 kPa (0 to 60 psig). The transesterification step is followed by a polymerization step where excess diol is removed and the oligomer converted further in a polycondensation reaction to yield a high molecular weight polyester. This method requires high diol:ester mole ratios (typically in the range of 1.5 to 6.0) to obtain high conversion of the alkyl ester groups in the transesterification step. High diol:ester ratios, however, require removal of excess diol in order to produce a polyester of sufficiently high molecular weight for film, sheet, and molded plastic applications. Removal of excess diol, however, is difficult when high-boiling diols are used, that is diols having a boiling point greater than about 230° C. such as, for example, CHDM. With high-boiling diols, it is frequently necessary to conduct the polycondensation step at high temperatures of about 270° C. to 310° C. and at pressures of about 53 kPa to about 0.013 kPa for extended periods of time in order to remove sufficient diol to produce a high molecular weight polyester.
Although diester-based polyester processes have been practiced commercially for many years, dicarboxylic acids are preferred as starting materials because water is generated as a by-product instead of an alcohol. Water is easy to remove and is non-hazardous. In addition, reacting a dicarboxylic acid directly with a diol eliminates the additional processing steps associated with the esterification of diacids to the corresponding dialkyl esters. Unfortunately, using dicarboxylic acids as a starting material instead of a dialkyl esters in polyester process presents a number of problems, especially when CHDM and other high boiling diols are used as a diol component. For example, many dicarboxylic acids have low solubility in diols which greatly reduces the rate of the esterification reaction. In particular, TPA has an low solubility in CHDM. To help overcome this low solubility, excess diol is often added to the esterifcation step to help drive conversion of the diol and the dicarboxylic acid to the oligomer. Any excess diol, however, must be removed during the polymerization step, often requiring extended process times at elevated temperatures and low pressures (typically less than 0.7 kPa). In addition, long reaction times during the polymerization step are often required to build sufficient molecular weight. Lengthy process times and elevated temperatures, however, frequently can lead to the development of color and the thermal degradation of the polyester. In polyester compositions that are substantially amorphous, this degradation can lead to a loss of clarity or to the formation of high color in the polyester product. For polyesters that exhibit a sufficient degree of crystallinity, the polycondensation reaction can be terminated at some intermediate step and the polyester then subjected to solid-state polymerization to increase molecular weight. Solid state polymerization, however, is expensive and requires additional equipment. These problems have created a need for a polyester process that avoids long reaction times and thermal degradation problems when dicarboxylic acids are used as starting materials in combination with CHDM.